Bioerodible Endoprosthesis

ABSTRACT

An endoprosthesis includes a plurality of struts defining a flow passage. At least one strut includes (a) a body comprising a bioerodible material and having a thickness and (b) a coating overlying the body. The coating includes a plurality of regions that allow physiological fluids to contact a plurality of corresponding areas of the underlying body when the endoprosthesis is implanted in a physiological environment. The plurality of regions are sized and arranged so that the contacted areas of the body erode substantially through the body in the thickness direction while the coating remains on the body when the endoprosthesis is implanted in the physiological environment.

TECHNICAL FIELD

This invention relates to bioerodible endoprostheses.

BACKGROUND

The body includes various passageways such as arteries, other bloodvessels, and other body lumens. These passageways sometimes becomeoccluded or weakened. For example, the passageways can be occluded by atumor, restricted by plaque, or weakened by an aneurysm. When thisoccurs, the passageway can be reopened or reinforced with a medicalendoprosthesis. An endoprosthesis is typically a tubular member that isplaced in a lumen in the body. Examples of endoprostheses includestents, covered stents, and stent-grafts.

Endoprostheses can be delivered inside the body by a catheter thatsupports the endoprosthesis in a compacted or reduced-size form as theendoprosthesis is transported to a desired site. Upon reaching the site,the endoprosthesis is expanded, e.g., so that it can contact the wallsof the lumen.

The expansion mechanism may include forcing the endoprosthesis to expandradially. For example, the expansion mechanism can include the cathetercarrying a balloon, which carries a balloon-expandable endoprosthesis.The balloon can be inflated to deform and to fix the expandedendoprosthesis at a predetermined position in contact with the lumenwall. The balloon can then be deflated, and the catheter withdrawn fromthe lumen.

In another delivery technique, the endoprosthesis is formed of anelastic material that can be reversibly compacted and expanded, e.g.,elastically or through a material phase transition. During introductioninto the body, the endoprosthesis is restrained in a compactedcondition. Upon reaching the desired implantation site, the restraint isremoved, for example, by retracting a restraining device such as anouter sheath, enabling the endoprosthesis to self-expand by its owninternal elastic restoring force.

It is sometimes desirable for an implanted endoprosthesis to erode overtime within the passageway. For example, a fully erodible endoprosthesisdoes not remain as a permanent object in the body, which may help thepassageway recover to its natural condition. Erodible endoprostheses canbe formed from, e.g., a polymeric material, such as polylactic acid, orfrom a metallic material, such as magnesium, iron or an alloy thereof.

SUMMARY

There is described an endoprosthesis that includes a plurality of strutsdefining a flow passage. At least one strut includes (a) a bodycomprising a bioerodible material and having a thickness and (b) acoating overlying the body. The coating includes a plurality of regionsthat allow physiological fluids to contact a plurality of correspondingareas of the underlying body when the endoprosthesis is implanted in aphysiological environment. The plurality of regions are sized andarranged so that the contacted areas of the body erode substantiallythrough the body in the thickness direction while the coating remains onthe body when the endoprosthesis is implanted in the physiologicalenvironment.

The body can include bioerodible material that erodes isotropicallyand/or anisotropically. The rate of erosion of the body in the thicknessdirection multiplied by the thickness of the body can be less than arate of erosion of the body along an interface between the body and thecoating multiplied by the distance between adjacent regions. In someembodiments, the rate of erosion of the body in the thickness directionmultiplied by the thickness of the body can be less than a rate oferosion of the body along an interface between the body and the coatingmultiplied by ½ of the distance between adjacent regions. For example,the plurality of regions can be arranged such that the distance betweenadjacent regions is equal to at least the thickness of the body (e.g.,at least twice the thickness of the body).

The bioerodible material of the body can have a first electric potentialand the coating has a second electric potential different from the firstelectric potential so that the body and the coating form a galvaniccouple when the endoprosthesis is implanted in a physiologicalenvironment. In some embodiments, the first electric potential can beless than the second electrode potential so that the body acts as ananode and the coating acts as a cathode when the endoprosthesis isimplanted in a physiological environment. For example, the body caninclude a bioerodible metal selected from the group consisting ofmagnesium, iron, zinc, and alloys thereof and the coating can include ametal selected from the group consisting of platinum, iridium, andalloys thereof. In other embodiments, the first electric potential canbe greater than the second electrode potential so that the body acts asa cathode and the coating acts as an anode when the endoprosthesis isimplanted within a physiological environment.

The bioerodible material of the body can be a bioerodible metal and/or abioerodible polymer. For example, the body can include a bioerodiblemetal selected from magnesium, iron, zinc, and alloys thereof and/or abioerodible polymer selected from polyglutamic acid, poly(ethyleneoxide), polycaprolactam, poly(lactic-co-glycolic acid), polysaccharides,and combinations thereof.

The coating can surround or partially surround the circumference of thebody. The coating can include a bioerodible material having a slowererosion rate than the bioerodible material of the body.

The regions of the coating can include voids, pores, and/or can have ahigher erosion rate than the remainder of the coating in a physiologicalenvironment.

The endoprosthesis, in some embodiments, can be a stent.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

DESCRIPTION OF DRAWINGS

FIGS. 1A-1D depict a first embodiment of an endoprosthesis strut.

FIGS. 2A-2D depict a second embodiment of an endoprosthesis strut.

FIGS. 3A-3D depict a third embodiment of an endoprosthesis strut.

FIGS. 4A-4D depict a third embodiment of an endoprosthesis strut.

FIG. 5 is a perspective view of an embodiment of an expanded stent.

FIGS. 6A-6C are longitudinal cross-sectional views illustrating deliveryof a stent in a collapsed state, expansion of the stent, and deploymentof the stent.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIGS. 1A-1D, 2A-2D, 3A-3D, and 4A-4D depict different of anendoprosthesis strut having a body 30 that includes a bioerodiblematerial and a coating 32 overlying at least a portion of the body 30.The coating 32 includes a plurality of regions 34 that expose aplurality of corresponding areas of the underlying body 30 to aphysiological environment when the endoprosthesis is implanted within aphysiological environment. As shown, the regions are simply voids in thecoating. In other embodiments, the regions 34 can be porous regionsand/or regions designed to erode prior to the remainder of the coating32. The regions 34 are sized and arranged so that the exposed areas ofthe body erode substantially through the body in the thickness directionbefore the coating 32 separates from the body 30. The spacing of theregions can account for an undercutting of the coating during theerosion of the body of the endoprosthesis, namely the erosion of thebioerodible body along the interface between the body and the coating.For example, the coating can separate from the body 30 due to thiserosion of the body 30 along the interface between the body 30 and thecoating 32. If the regions are spaced too closely, this undercutting canresult in a premature separation of the coating from the body.

The arrangement of regions, particularly the spacing between adjacentregions, is determined, in part, based on the thickness of the body andthe erosion characteristics of the body 30 (e.g., whether the body 30erodes isotropically or anisotropically). For a body 30 that erodesisotropically (i.e., homogeneously in all directions), the spacing Sbetween adjacent regions can be at least equal to the thickness T of thebody. For example, as shown in FIGS. 1C, 1D, 2C, 2D, 3C, and 3D, a body30 that erodes isotropically can produce undercuts 36 beneath the edgesof the regions 34 of the coating 32. For embodiments where the coating32 is only on one side of the body 30, such as shown in FIGS. 1A-1D, athickness T can be less than or equal to the spacing S between adjacentregions 34 to ensure that the body erodes substantially through thethickness T before the coating 32 separates from the body 30. In someembodiments, the body can be formed of a single bioerodible materialhaving consistent erosion properties. In other embodiments, the body canbe a composite of a plurality of bioerodible materials and can havevarying erosion properties. The size and arrangement of the regions inthe coating can depend on the overall erosion properties of the body.

As shown in FIGS. 1C and 1D, an uncoated opposite side 38 of the body 30can erode in a substantially uniform manner across its surface, and atsubstantially the same rate as the body erodes inward in the thicknessdirection from the exposed areas of the regions 34. Having a thickness Tthat is less than the spacing S can ensure that the body 30 erodessubstantially through the thickness, from both sides of the body, beforeadjacent undercuts from adjacent regions 34 erode into each other. Insome embodiments, however, the placement of the endoprosthesis within aphysiological environment can alter the rate of erosion of particularportions and/or sides of the endoprosthesis. For example, a side of anisotropic body 30 placed against a vessel wall may erode at a slower orfaster rate than a portion of the body 30 positioned adjacent to flowingblood, depending on the characteristics of the material(s) of the body30. The pH of the physiological environment can alter the erosion rateof some bioerodible materials and the erosion of various bioerodiblematerials can alter the pH of the surrounding fluids and/or tissues. Theendoprosthesis can be designed so that the contacted areas of the bodyerode substantially through the body in the thickness direction whilethe coating remains on the body even with these variations to theerosion rate of the body.

The body 30 can also include coatings on selected portions of thesurface of the body (e.g., on more than one side). In some embodiments,the coating 32 can surround the entire circumference of the body 30. Inother embodiments, the coating can extend along the length of the body30 in selected portions around the circumference of the body 30. Forexample, as shown in FIGS. 3A-3C, the body can have coatings on oppositesides of the body 30, but have adjacent sides of the body remainexposed. The coatings on portions (or sides) of the body can haveuniform or non-uniform patterns of regions. For example, as shown inFIGS. 2A-2B, a coating 32 can have identical patterns on opposite sidesof the body and have adjacent sides without the presence of any regions.As shown, the regions of the opposite sides are aligned. In otherembodiments, the regions can be offset. In some embodiments, oppositesides can have completely different arrangements of regions, differentsizes of regions, and/or different spacing of regions. Although the body30 is shown as having a rectangular cross-sectional shape, the body canhave other cross-sectional shapes (e.g., circular or polygonal) and canhave a non-constant cross-sectional shapes, thicknesses, and/or widths.

An isotropically eroding body 30 having identical and aligned patternsof regions on opposite sides of the body 30, as shown in FIGS. 2A-2D,can have a thickness T of less than or equal to the spacing S betweenadjacent regions 34. As shown in FIG. 2D, the regions 34 of oppositesides of the body 30 allow for erosions paths through the thickness ofthe body 30 that meet at the approximate center of the body 30, whichprovides an erosion path through the thickness of the body 30 prior tothe separation of the coating 32 from the body 30 along the interfacebetween the body 30 and the coating 32. A body 30 having differentpatterns of regions 34 on opposite sides of body 30 can allow fordifferent maximum thicknesses depending on an amount of time requiredfor erosion paths on opposite sides to intersect (if at all). If erosionpaths do not intersect, for example, if body 30 has a region-freecoating 40 on the opposite side 38 of the body 30 as shown in FIGS.3A-3D, the thickness T of the body can be less than or equal to half ofthe spacing S between adjacent regions. As shown in FIG. 3D, the body 30can erode through its thickness T prior to the separation of the coating32 from the body 30. For example, iron and magnesium erodeisotropically.

A body 30 that erodes anisotropically can allow for differentarrangements of regions in the coating 32. The arrangement, however,must account for the different rates of erosion of the body in eachdirection. For example, as shown in FIGS. 4A-4D, an endoprosthesishaving a body 30 that erodes anisotropically can allow for a closerspacing of adjacent regions 34 if the body 30 erodes at a faster rate inthe thickness direction than in directions parallel with the coating.FIGS. 4C and 4D depict a preferred erosion direction E that results in afaster erosion rate in the thickness direction. In other embodiments,the preferred erosion direction E can be parallel to the thicknessdirection. In embodiments having a non-coated opposite underside 38 ofthe body as shown in FIGS. 4A-4D, the thickness T of the body can beless than or equal to the minimum spacing between the plurality ofregions multiplied by the average rate of erosion of the body along aninterface of the body and the coating multiplied by divided by the rateof erosion of the body in the thickness direction. The same can apply toan anisotropic body having coatings with regions 34 on opposite sides ofthe body where the regions on opposite sides are aligned along an axisparallel to the direction of preferred erosion E. In other embodiments,such as those having an region-free coating 40 on the opposite side(e.g., the underside 38) of the body 30, the thickness can be up to 50%of the minimum spacing between the plurality of regions multiplied bythe average rate of erosion of the body along an interface of the bodyand the coating divided by the rate of erosion of the body in thethickness direction. For example, some bioerodible polymers erodeanisotropically.

The size, spacing, and arrangement of the regions can also control thesize of the particles dispensed into the surrounding body fluid. Asshown in FIGS. 1D, 2D, 3D, and 4D, different arrangements of the regionscombined with different body characteristics can impact the size andshape of the pieces of the eroding endoprosthesis that separate from thereminder of the endoprosthesis once a path erodes through the thicknessdirection of the body and the coating 32 separates from the body. Insome embodiments, the arrangement of regions can vary to ensure thatparticular parts of a body of an endoprosthesis separate from theremainder of the endoprosthesis in a particular order.

The body 30 includes a bioerodible material (e.g., a bioerodible metal,a bioerodible polymer, a bioerodible ceramic, and/or a bioerodible metalsalt). Examples of bioerodible metals suitable for use in the body 30include magnesium, iron, zinc, and alloys thereof. An example of asuitable bioerodible iron alloy includes Fe-35 Mn. Examples ofbioerodible polymers suitable for use as the body 30 includepolyglutamic acid, polylactic acid (PLA), poly(ethylene oxide) (PEO),poly-serine, polycaprolactam, poly(lactic-co-glycolic acid) (PLGA),cyclodextrins, polysaccharides (e.g., chitosan and hyaluronan),copolymers thereof, and combinations thereof. Other examples ofbioerodible polymers include polyglycolic acid (PGA), polycaprolactone(PCL), polyorthoesters, polydioxanone, poly(trimethylene carbonate)(PTMC), polyphosphazenes, polyketals, proteins (e.g., glycoproteins,fibrin, collagen, gelatin, pectin), polyanhydrides (e.g., poly(esteranhydride)s, fatty acid-based polyanhydrides, amino acid-basedpolyanhydrides), polyesters, polyester-polyanhydride blends,polycarbonate-polyanhydride blends, and/or combinations thereof. Thebioerodible polymers can be blended and/or copolymerized to alter thedegradation characteristics.

The coating 32 can include a biocompatible material that can protect theunderlying material from erosion. The portion of the coating 32 thatseparates the plurality of regions 34 can protect the underlying bodyfrom contact with physiological fluids at least until the contactedareas of the body erode substantially through the body in the thicknessdirection. For example, the portion of the coating 32 that separates theplurality of regions 34 can be non-porous. The coating 32 can includebioerodible materials and/or more stable materials. A coating of abioerodible material can have a slower erosion rate than the body 30.The thickness of the coating 32 can be such that at least a portion ofthe coating remains non-eroded and on the body 30 at least until thecontacted areas of the body erode substantially through the body in thethickness direction. The coating 32 can be between 10 nanometera and 10microns thick. Examples of suitable bioerodible materials for use in thecoating include bioerodible polymers, bioerodible metals, biologicalmaterials, and combinations thereof. Suitable bioerodible metals includeiron, zinc, and alloys thereof. Suitable bioerodible polymers caninclude polyglutamic acid, polylactic acid, poly(ethylene oxide),poly-serine, polycaprolactam, poly(lactic-co-glycolic acid),cyclodextrins, polysaccharides, copolymers thereof, and combinationsthereof. The bioerodible polymers can be blended to alter thedegradation characteristics. Suitable biological materials can includecollagen, hyaluronic acid, glycoproteins, polysaccharides, pectin, andcombinations thereof. In some embodiments, the coating 32 and the body30 can form a galvanic couple that can allow for the preferentialerosion of the body 30 (e.g., the body can act as an anode while thecoating acts as a cathode). For example, a magnesium body can have aniron coating so that the magnesium body erodes preferentially.

A coating 32 can also include more stable materials, which can beselected from polymers, metals, ceramics, salts, and biologicalmaterials. Examples of relatively stable metals suitable for use in thecoating 32 include: tantalum, titanium, cobalt, chromium, stainlesssteel, cobalt-chromium alloys, platinum enhanced stainless steel alloys,Nitinol alloys, and noble metals, such as platinum, palladium, iridium,and ruthenium. Suitable ceramics can include, for example, CrOx, AlOx,ZrOx, SiOx, TiNOx, and oxides of noble metals such as IrOx. Suitablepolymers can included SIBS and PVDF. Suitable biologic materials caninclude collagen, fibrin, alginates, and polysaccharides. A relativelystable coating 32 can provide a firm substrate to an otherwise erodingstructure, thus facilitating endothelial cell growth and/or attachmentwhile retaining sufficient flexibility to facilitate delivery anddeployment of the endoprosthesis. Moreover, the visibility of theendoprosthesis to imaging methods, e.g., X-ray and/or Magnetic ResonanceImaging (MRI), can be enhanced, even after the endoprosthesis is partlyeroded, by e.g., incorporating a radiopaque material into the coating32.

The regions 34 allow for a plurality of corresponding areas of theunderlying body to be exposed to a physiological environment when theendoprosthesis is implanted in a physiological environment. As shown inFIGS. 1A-1D, 2A-2D, 3A-3D, and 4A-4D, the regions 34 are merely squareshaped voids in the coating. The regions 34, however, can have othershapes (e.g., circular, elliptical, polygonal, etc). The size and/orshape of different regions 34 of a single coating 32 can be different orthe same. In other embodiments, not shown, the regions 34 can includepores that allow the underlying areas of the body 30 to be contacted byphysiological fluids when the endoprosthesis is implanted into aphysiological environment. These regions of pores can be surrounded byother portions of the coating 32 that do not allow the underlyingsurface of the body to be contacted by physiological fluids. Forexample, a coating 32 having porous regions can be produced byimplanting ions in selected regions of the coating, followed by leachingor burning the ions out to create porosity in those selected regions(regions 34). The regions 34, can also include areas of the coatingdesigned to erode to expose underlying areas of the body 30 prior to theerosion of the remainder of the coating 32. The spacing between theseregions 34 of faster erosion allow the underlying areas of the body toerode substantially through the body in the thickness direction while atleast a portion of the coating remains on the body. For example, theregions 34 can have a substantially thinner thickness than the remainderof the coating, can include a material that erodes at a faster rate thanthe remainder of the coating, and/or can include a nano/micron scaleroughening of the coating that accelerates the erosion rate of theregions 34 versus other portions of the coating 32.

The coating 32 can be deposited on the body 30 of the endoprosthesis byconventional coating techniques or can be created on the surface of abody 30 by modifying the surface of the body 30. Conventional masking,lithographic, and templating techniques can be used to control theplacement of the regions 34 in the coating 32. For example, a coating 32can be produced with a mask made of a set of fine wires or a mesh of adesired pattern. The mask can be set against the inner and/or outerdiameters of an endoprosthesis (e.g., a stent), a dense coating of thedesired materials can be deposited, and the mask removed.

Lithographic techniques can include soft lithography or nano-sphere (ormicro-sphere) lithography. Soft lithography is well suited for nonplanarsurfaces and can include techniques such as microcontact printing,replica molding, microtransfer molding, micromolding in capillaries(“MIMIC”), and solvent assisted micromolding (“SAMIM”). Microcontactprinting can use a PDMS stamp to print a single molecule thick patternof ink molecules on a surface of an endoprosthesis. A desired coatingmaterial can then be deposited on the surface and a lift process canthen be used to remove the coating applied to the inked areas of theendoprosthesis. This can generate a surface including regions of adesired size, spacing, and arrangement corresponding to the printedpattern. The PDMS stamp can be made by conventional photolithography.

Nano-sphere lithography is an effective way to grow periodic andlarge-area nano-structures. Nano-sphere lithography uses self-assemblednano-spheres (e.g., polystyrene) as a template followed by a depositionprocess to deposit coating materials in the void spaces between portionsof the template. The nanosphere template can be deposited on anendoprosthesis surface by drop coating, spray coating, spin coating,self assembly, sedimentation in a force field, or via crystallization.The nano-spheres can be held to the surface by Van der Waals forces,electro static forces, a thin adhesive layer, and/or plating a thinlayer of nickel, titanium, platinum chromium to secure the particles tothe stent surface. The deposited template nano-spheres generallyassemble in a close packed fashion. The spheres can be isolated byreactive ion etching of the spheres, after deposition, to reduce thesphere diameter and hence isolate the nano-spheres. Once the templatehas been dried, the void spaces between the templated spheres can befilled with a variety of metals and oxide materials throughelectrochemical deposition, e.g., by physical vapor deposition (“PVD”)or chemical vapor deposition (“CVD”). The void spaces can also be filledwith liquid precursor(s) of one or more polymers, sol-gel precursors ofa ceramic material, a solution containing an inorganic salt, and/or adispersion of nano metal or oxide particles to form the coating 32. Thecoating is formed such that the spheres are partially exposed. Once thecoating material is deposited, the spheres can be removed by calcinationin air or by dissolution in a chemical solution. Additionally, thespheres can be mixed with biocompatible materials, such as silica ortitania, prior to deposition, and the spheres removed prior to thedeposition of the coating to produce porous regions surrounded by areasof the coating that protect the underlying body 30 surface area fromerosion.

Bioerodible materials can also be used to template the surface of a bodyof an endoprosthesis. For example, a bioerodible polymer can bedeposited onto select regions of the surface of an iron endoprosthesisthat correspond to the regions 34 to be formed once the coating materialis deposited. The coating material can then be deposited onto theremaining exposed iron surfaces of the endoprosthesis. In otherembodiments, the remaining exposed surfaces of the iron endoprosthesiscan be modified to create the coating 32. The bioerodible polymer canthen be removed. In some embodiments, the bioerodible polymer can remainpresent as part of the regions 34 and allowed to erode once theendoprosthesis is implanted in a physiological environment. Thebioerodible polymer left in the regions 34 to erode in a physiologicalenvironment, can include one or more therapeutic agents.

Physical vapor deposition (“PVD”) can be used to deposit the coatingmaterial. For example, a coating 32 of biocompatible materials, such asiridium oxide, tantalum, titanium, and/or titanium-oxy-nitride, can bedeposited onto a bioerodible body, such as magnesium and/or iron, by PVDtechniques. The use of PVD techniques can allow the precise placement ofregions 34. PVD techniques can also create a coating where the regions34 are select regions of the coating 32 having a porosity that allows acorresponding area of the body to be exposed to a physiologicalenvironment when the endoprosthesis is implanted while the surroundingareas of the coating 32 protect the underlying body surface area fromerosion. In some embodiments, a modified Holistic Process PerformanceMeasurement System (“mHPPMS”) PVD process can be used to deposit coating32 onto the body 30 to create a strong bond between the body 30 and thecoating 32. A discussion of HPPMS can be found in the followingarticles, which are hereby incorporated by reference: U. Krausea, M.Lista & H. Fuchsb, Requiremens of Power Supply Parameters for High-PowerPulsed Magnetron Sputtering, 392 Thin Solid Films 196-200 (2001) & S.Konstantinidis, J. P. Dauchot & M. Hecq, Titanium Oxide Thin FilmsDeposited by High-Power Impulse Magnetron Sputtering, 515 Thin SolidFilms 1182-1186 (2006).

Co-deposition processes can also be used to deposit a coating 32 havingregions 34, including a material having a faster erosion rate than theremainder of the coating 32. For example, a relatively stable material,such as tantalum, cobalt, and/or chromium, can be deposited with abioerodible material, such as magnesium, to create regions 34 includinga higher percentage of magnesium than the remainder of the coating 32.Once the magnesium of the composite coating 32 erodes, the regions 34can allow for the erosion of the underlying material of the body, whilethe remainder of the coating 32 continues to protect the underlyingsurface area of the body from erosion. For example, the erosion of themagnesium can leave fine pores through the coating 32 in the areas ofthe regions 34.

The surface of a bioerodible endoprosthesis can also be modified tocreate a coating 32. For example, alloying materials can be implanted toproduce a relatively stable and/or more slowely eroding material on thesurface of an iron, magnesium, or zinc containing endoprosthesis. Forexample, alloying materials can be implanted on the surface of an ironstent to create a thin coating of stainless steel.

The body 30 and the coating 32 can, in some embodiments, includematerials that form a galvanic couple between the body and the coatingwhen the endoprosthesis is implanted in a physiological environment. Thebody and the coating can be electrically conductive to ensure that thematerials of the galvanic couple remain in electrical contact with eachother. The galvanic couple, in the presence of ion-containing fluids,such as plasma and/or blood, forms an electrochemical cell in which thebody 30 and the coating 32 act as electrodes and the fluid acts as anion-conducting electrolyte. The galvanic couple can be formed betweenmetals, electrically conductive polymers (e.g., polyvinylidene fluoride,polyaniline, and the like), and electrically conductive polymercomposites (e.g., polymer matrices containing electrically conductiveparticles, wires, meshes, or the like).

The galvanic couple can impact the rate of erosion of overallendoprosthesis, or a portion thereof. For example, a body 30 having amaterial acting as the galvanic anode and a coating 32 acting as agalvanic cathode can allow for a preferential erosion of the body (e.g.,a body 30 including magnesium, zinc, and/or iron with a coating 32including platinum and/or iridium). The greater the difference inelectric potential between the materials of the body and the coating,the greater the preferential erosion of the body. Alternatively, a bodyhaving a bioerodible material acting as a cathode and a relativelystable coating acting as an anode can result in a reduced erosion ratefor the body.

The endoprosthesis includes a plurality of struts. The struts define aflow passage. One or more of the struts can include a body 30 having acoating 32, as described above. In some embodiments, the endoprosthesiscan include a plurality of different struts bodies 30 of differentbioerodible materials and/or coatings having regions of different sizes,spacing, arrangements, and/or of different coating materials. Forexample, the endoprosthesis can be a stent. Referring to FIG. 5, theendoprosthesis can be in the form of a balloon-expandable stent 20. Thestent body is in the form of a tubular member defined by a plurality ofstruts (e.g., the bands 22 and connectors 24). The connectors 24 extendbetween and connect adjacent bands 22. During use, bands 22 can beexpanded from an initial, smaller diameter to a larger diameter tocontact the stent 20 against a wall of a vessel, thereby maintaining thepatency of the vessel. Connectors 24 can provide the stent 20 with theflexibility and conformability that allow the stent to adapt to thecontours of the vessel. The stent 20 defines a flow passage therethroughand is capable of maintaining patency in a blood vessel. In otherembodiments, the endoprosthesis can be in the form of a self-expandingstent.

The stent 20 can, in some embodiments, include a plurality of differentstruts having bodies 30 of different bioerodible materials and/orcoatings having regions of different sizes, spacing, arrangements,and/or of different coating materials. For example, an iron stent canhave different struts having different coatings 32 over bodies of iron.One coating can have a pattern of regions designed to decrease theoverall erosion rate of the strut relative to the remainder of the stent20. Additionally, one or more struts could include a coating of amaterial to increase the overall erosion rate of the one or more strutsrelative to uncoated iron struts. For example, the erosion rateincreasing coating can act as a galvanic cathode to increase the erosionrate of the strut. Each coating can include a coating material and apattern of regions designed to effect a particular erosion rate for eachstrut. For example, connectors 24 can be designed to erode prior to theerosion of the bands 22. In other embodiments, selected regions of thestent 20, each including a plurality of bands and connectors, can havedifferent coatings 32, and perhaps select regions without any coating,to produce a stent 20 that erodes to produce a shorter stent 20 prior tocomplete erosion.

Stent 20 can be of any desired shape and size (e.g., superficial femoralartery stents, coronary stents, aortic stents, peripheral vascularstents, gastrointestinal stents, urology stents, and neurology stents).Depending on the application, the stent can have a diameter of between,for example, 1 mm to 46 mm. In certain embodiments, a coronary stent canhave an expanded diameter of from 2 mm to 6 mm. In some embodiments, aperipheral stent can have an expanded diameter of from 5 mm to 24 mm. Incertain embodiments, a gastrointestinal and/or urology stent can have anexpanded diameter of from 6 mm to about 30 mm. In some embodiments, aneurology stent can have an expanded diameter of from about 1 mm toabout 12 mm. An Abdominal Aortic Aneurysm (AAA) stent and a ThoracicAortic Aneurysm (TAA) stent can have a diameter from about 20 mm toabout 46 mm.

Stents can be used, e.g., delivered and expanded, using a catheterdelivery system. Catheter systems are described in, for example, WangU.S. Pat. No. 5,195,969, Hamlin U.S. Pat. No. 5,270,086, andRaeder-Devens, U.S. Pat. No. 6,726,712. Stents and stent delivery arealso exemplified by the Sentinol® system, available from BostonScientific Scimed, Maple Grove, Minn. Referring to FIGS. 6A-6C, aballoon-expandable stent 20 can be placed over a balloon 12 carried neara distal end of a catheter 14, and directed through the lumen 16 (FIG.6A) until the portion carrying the balloon and stent reaches the regionof an occlusion 18. The stent 20 is then radially expanded by inflatingthe balloon 12 and compressed against the vessel wall with the resultthat occlusion 18 is compressed, and the vessel wall surrounding itundergoes a radial expansion (FIG. 6B). The pressure is then releasedfrom the balloon and the catheter is withdrawn from the vessel (FIG.6C).

In some embodiments, stents can also be a part of a covered stent or astent-graft. In other embodiments, a stent can include and/or beattached to a biocompatible, non-porous or semi-porous polymer matrixmade of polytetrafluoroethylene (PTFE), expanded PTFE, polyethylene,urethane, or polypropylene.

In some embodiments, stents can be formed by fabricating a wire having abioerodible body and a coating including a plurality of the abovedescribed regions, and knitting and/or weaving the wire into a tubularmember.

In some embodiments, stents can include therapeutic agents incorporatedinto one or more portions of the body 30 and/or the coating 32(including the regions 34). Stents can also include additional drugeluding layers and/or deposits of therapeutic agents.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of this disclosure. Accordingly, other embodimentsare within the scope of the following claims.

1. An endoprosthesis comprising a plurality of struts defining a flowpassage, at least one strut comprising: (a) a body comprising abioerodible material having a thickness, and (b) a coating overlying thebody, the coating comprising a plurality of regions that allowphysiological fluids to contact a plurality of corresponding areas ofthe underlying body when the endoprosthesis is implanted in aphysiological environment, the plurality of regions being sized andarranged so that the contacted areas of the body erode substantiallythrough the body in the thickness direction while the coating remains onthe body when the endoprosthesis is implanted in the physiologicalenvironment.
 2. The endoprosthesis of claim 1, wherein the bioerodiblematerial erodes isotropically.
 3. The endoprosthesis of claim 2, whereinthe plurality of regions are arranged such that the distance betweenadjacent regions is equal to at least the thickness of the body.
 4. Theendoprosthesis of claim 2, wherein the plurality of regions are arrangedsuch that the distance between adjacent regions is equal to at leasttwice the thickness of the body.
 5. The endoprosthesis of claim 1,wherein the bioerodible material erodes anisotropically.
 6. Theendoprosthesis of claim 1, wherein a rate of erosion of the body in thethickness direction multiplied by the thickness of the body is less thana rate of erosion of the body along an interface between the body andthe coating multiplied by the distance between adjacent regions.
 7. Theendoprosthesis of claim 1, wherein a rate of erosion of the body in thethickness direction multiplied by the thickness of the body is less thana rate of erosion of the body along an interface between the body andthe coating multiplied by ½ of the distance between adjacent regions. 8.The endoprosthesis of claim 1, wherein the bioerodible material has afirst electric potential and the coating has a second electric potentialdifferent from the first electric potential so that the body and thecoating form a galvanic couple when the endoprosthesis is implanted in aphysiological environment.
 9. The endoprosthesis of claim 8, wherein thefirst electric potential is less than the second electrode potential sothat the body acts as an anode and the coating acts as a cathode whenthe endoprosthesis is implanted in a physiological environment.
 10. Theendoprosthesis of claim 8, wherein the bioerodible material of the bodyis a bioerodible metal selected from the group consisting of magnesium,iron, zinc, and alloys thereof and the coating comprises a metalselected from the group consisting of platinum, iridium, and alloysthereof.
 11. The endoprosthesis of claim 8, wherein the first electricpotential is greater than the second electrode potential so that thebody acts as a cathode and the coating acts as an anode when theendoprosthesis is implanted in a physiological environment.
 12. Theendoprosthesis of claim 1, wherein the bioerodible material of the bodyis a bioerodible metal selected from the group consisting of magnesium,iron, zinc, and alloys thereof.
 13. The endoprosthesis of claim 1,wherein the body comprises a bioerodible polymer selected from the groupconsisting of polyglutamic acid, poly(ethylene oxide), polycaprolactam,poly(lactic-co-glycolic acid), polysaccharides, and combinationsthereof.
 14. The endoprosthesis of claim 1, wherein the coatingsurrounds the circumference of the body.
 15. The endoprosthesis of claim1, wherein the coating partially surrounds the circumference of thebody.
 16. The endoprosthesis of claim 1, wherein the regions comprisevoids in the coating.
 17. The endoprosthesis of claim 1, wherein theregions comprise pores in the coating.
 18. The endoprosthesis of claim1, wherein the regions have a higher erosion rate than the remainder ofthe coating in a physiological environment.
 19. The endoprosthesis ofclaim 1, wherein the coating comprises a bioerodible material having aslower erosion rate than the bioerodible material of the body.
 20. Theendoprosthesis of claim 1, wherein the endoprosthesis is a stent.